Many women are now putting off having babies until they are in their late thirties or early forties so as to enable them to build up their careers and buy the expensive house, before being trapped by bringing up a family.
Motherhood in near middle age is still frowned on by the medical profession, but it is not unusual. The forty-something mums Cherie Blair will be joining include Jerry Hall, who this week was photographed with the Blair's neighbour, Gordon Brown, at a charity event in Downing Street, and the actress Patricia Hodge.
Tony can be counted on to play the role of doting father, who is likely to share in the ante-natal classes, and may even be there to hold her hand at the birth. Tony has increasingly allowed the lack of a normal family life for his other "kids" to grow on his mind.
"I would love to quit Number Ten for a family home," he once told a reporter. "We would prefer to live in our own home but it is not possible for security reasons. Home for me is where the family is. My kids are very easy. They are normal kids who have normal lives."
Like most Nineties fathers, he admits to being outplayed by the children at computer games, and to learning from them all about the Internet.
He made plain before flying out to South Africa for the Commonwealth Heads of Government Conference that he would be far happier spending the weekend "with the kids" than with the Queen, Thabo Mbeki and the other leaders of the Commonwealth.
Given the spotlight that is inevitably turned on the Prime Minister's children, the Blair "kids" - Euan, 15, Nicholas 14 and 11-year-old Kathryn appear to be remarkably well-adjusted, and "normal".
Euan and Nicholas like nothing better than a kickabout with their dad, who managed to keep the ball in the air in a heading stunt with Kevin Keegan a few years ago but the Blairs have made a point of trying to protect their children from exposure by the media.
The controversy over their choice of the Oratory School for Euan, rather than the local comprehensive in Islington, before the Blairs moved to Downing Street was followed by a bigger row this year when they found a place for Kathryn at the Sacred Heart High School. It was six miles from Whitehall and the Mail on Sunday claimed that they had received preferential treatment, angering some parents whose children had been rejected by the school. The Blairs took the unusual step of lodging a formal complain with the Press Complaints Commission, which produced a ground-breaking judgement in July which in future may protect the children of other rich and famous parents from intrusive reporting.
However, the Blairs have never tried to keep their children completely out of the view of the cameras. They were seen together on their family holiday last January in the Seychelles and again for this summer's holiday near Pisa, when the Italian authorities caused an international scene by clearing the beach so that the Blairs at play would not be disturbed by the paparazzi.
Adding a fourth child to their family may cause more than the usual disruption in Downing Street. The Blairs gave up their half-million-pound family house in Islington to move above the "shop" when dad became Prime Minister, but they quickly realised that the flat allocated to Prime Ministers in the past, including John Major and Margaret Thatcher, was totally inadequate for a young family. They did a swop with the Chancellor of the Exchequer and now live above Number Eleven, but having a baby in Downing Street could make the cramped accommodation more difficult for both the Blairs and Mr Brown.
The sleepless nights that go with parenthood may also make Tony's day job seem even more stressful, and the bags under the eyes that have grown deeper since May 1997, when the Blairs arrived to the gates of power with the children, are bound to become darker.
Cherie is also likely to have to give up her day job as a highly respected lawyer, specialising in employment law, to look after her baby, although the Nineties have seen the advent of the Nanny as the must-have ad-on to the perfect middle class family. The Blairs, given the staffing arrangements for the Prime Minister, are likely to be spared the need to build up points in a baby sitting cooperative. Mr Brown may be left looking after the baby for them.